1. Department of Obstetrics, Jining First People's Hospital, Jining, Shandong 272000, P.R. China
2. Department of Anesthesiology, People's Hospital of Zhangqiu District, Jinan, Shandong 250200, P.R. China
3. Department of Surgery, People's Hospital of Zhangqiu District, Jinan, Shandong 250200, P.R. China
4. Department of Operation Room, People's Hospital of Zhangqiu District, Jinan, Shandong 250200, P.R. China